Megu Ohtaki
Department of Environmetrics and Biometrics, Research Institute for Radiation Biology and Medicine, Hiroshima University

Abstract

So-called 'Black Rain', which might include radioactivity, fell around the western part of Hiroshima City and the northwest suburbs for several hours just after the explosion of the atomic bomb on August 6, 1945. In those days, there was only one official weather station in the neighborhood of Hiroshima City, the Eba meteorological observatory, which was located 3.7 km southwest from the hypocenter. Therefore, a questionnaire survey is the only way to grasp the actual situation of spatial-time distribution of 'Black Rain'. In 2008, Hiroshima City carried out a questionnaire survey of about 37,000 inhabitants of Hiroshima and its suburbs who might have experienced 'Black Rain', investigating the start time, end time, and location of the rain. Nonparametric smoothing based on a local linear regression model revealed the spatial-time distribution of 'Black Rain', which began around the western suburb of Hiroshima at about 9:00 a.m., became heaviest at about 10:00 a.m. spreading toward the northwest direction, and disappeared at around 30 km north-northwest from the hypocenter at about 3:00 p.m. on August 6, 1945. The estimated rainy area is about five to six times wider than Uda's 'heavy rain area', in which only those who had experience of 'Black Rain' have received medical support from the Japanese government.